Carbon black, which is made by the pyrolytic decomposition of hydrocarbon feedstocks such as oil in a furnace process, strongly influences the properties of rubber compositions containing such carbon black. It is known in the art that various after treatments of carbon black influence the properties of rubber composition containing such carbon blacks. It has been reported, for instance, that heating a furnace-type black in contact with aqueous nitric acid at a high temperature will increase the diphenylguanidine absorption value of the carbon black. Other references show that oxidation of carbon black with nitric acid lowers the modulus of the rubber composition containing such carbon black.
The specification for the carbon black by the carbon black consumer contains usually the two properties modulus and scorch time. The modulus is a measure for the force per unit of cross-section necessary to elongate a sample of rubber to a certain elongation such as 300%. The scorch time is a measure that relates to the curing properties of the carbon black. Generally a low scorch time relates to a fast curing rubber and a high scorch time correspondingly to a slow curing rubber. During the carbon black production, the modulus and the scorch time are determined by compounding the carbon black produced with rubber and determining the properties defined. It sometimes occurs that the carbon black as produced, mixed with rubber, result in a composition having a too high modulus and/or a too low scorch time as compared to the specification and as such is considered "off-specification". It would, therefore, be desirable to have a process available by which such off-specification carbon black resulting in a too high modulus and/or a too short scorch time can be brought back to the specification.
In the prior art after treatment of carbon black with nitric acid, the nitric acid is employed in the range of concentration of 5 wt. % in water or more. The nitric acid treatment both adds to the cost of the ultimate carbon black product and introduces handling and corrosion problems. It would, therefore, be desirable to have a process available by which both the cost mentioned and the handling problems described could be reduced.